Add cimport support

Created by: papr

Attempt to implement #716 (closed)

Test setup:

pip install git+https://github.com/romanroibu/pupil-core-pyav-extension@a6e28fd0fd089af08182be1ca151084bc63beb03
python -c "import pupil_av.test"

pupil_av/test.pyx content:

from av.buffer cimport Buffer

cdef Buffer test_buffer = Buffer()
print(test_buffer)

Therefore the expected output of the test setup is <av.buffer.Buffer object at 0xaddress>.

In order for this to work, pupil-core-pyav-extension requires av to be a build requirement (defined in pyproject.toml) and to be a runtime requirement (defined in setup.cfg). Currently, both point to this PR.

To be fixed

• Currently, this PR only includes pxd files that are part of the av folder. It would be great if we found a solution to include the include/*.pxd files as well.